THE SOIL AND SOIL FORMATION 



called adhesion. The particles of fine clay being several 

 hundred times smaller than the particles of coarse sand, 

 there is presented to the water a very much larger total 

 area of surfaces in a given bulk of clay soil than in sand ; 

 and, therefore, the total area of the surface films is much 

 greater. In case of a soil filled with ground water, as in 

 Figure 26, where the drain is opened, the movement is 

 downward in response to gravitation. When all this 

 hydrostatic or ground water has 

 been drained out, leaving the soil 

 with its capillary power all satis- 

 fied, the films covering the par- 

 ticles are relatively thick. As the 

 soil dries out from evaporation 

 and through removal by the roots 

 of plants, this film becomes 

 thinner, until only the hygro- 

 scopic water that which the air 

 cannot take away remains. 

 When the films are thick, the 

 capillary water moves rather 

 freely toward a point where a 

 root is exhausting the supply; 

 while, where the film is thin, there 

 are many more openings, or 

 spaces, in the soil, and the move- 

 ment of water is much retarded 



capillarity through the wick is un- 1 J , ... . ., t . ,, , 



able to pass this opening where by the mCtlOn in the thin FllmS. 

 the capillary connections are sepa- * 



rate d- The movement of water and 



plant food toward the growing roots of plants, however, 

 is not so rapid as many suppose. 



Other facts concerning this interesting force are shown 

 in Figures 32 and 33. A new lamp wick, Figure 32, is 

 attached to an old one, which has become too short to 

 reach the oil, but the threads connecting the two ends 

 are not drawn tightly, leaving the two ends slightly 



